1. Field of the Invention
The invention relates to a DC-AC converter (hereinafter referred to as an “inverter”) for producing an AC voltage to be used for driving a load from a DC power source such as a mains adapter, which is an accessory of an electrical appliance, or a battery, as well as to an AC power supply method.
2. Description of the Related Art
A cold cathode fluorescent lamp (CCFL) has come into use as a back light source for a liquid-crystal monitor of a notebook PC or a liquid-crystal display of a liquid-crystal TV receiver and so on. In general, the CCFL might be of higher efficiency and longer life use than those of an ordinary hot cathode fluorescent lamp. A filament usually provided in the hot cathode fluorescent lamp is omitted from the CCFL.
Start-up and activation of the CCFL require a high AC voltage. For instance, a start-up voltage is about 1,000 volts, and an operating voltage of the CCFL is about 600 volts. Such a high AC voltage is produced from a DC power source of a laptop computer or a liquid-crystal TV receiver through use of an inverter.
In conventional arts, Royer circuit has been known for an inverter use of a CCFL. The Royer circuit is formed from a saturable magnetic core transformer, a control transistor, and other circuit elements. The Royer circuit can perform a self-oscillation by means of non-linear permeability of the saturable magnetic core transformer and a non-linear current gain characteristic of the control transistor. However, the Royer circuit itself does not need any external clock or driver circuit.
The Royer circuit is basically a constant-voltage inverter. Therefore, when fluctuation arises in an input voltage or a load current, the Royer circuit cannot maintain a constant output voltage. Consequently, a regulator is required for supplying the power to the Royer circuit. In this regard, it was difficult for an inverter using the Royer circuit to be down-sized and achieve a efficient low power conversion.
Among other related arts, there has been proposed an inverter using a center-tap-type transformer, in which the transformer has a primary winding which supplies a DC voltage to a center tap, and a secondary winding for outputting an AC voltage such as disclosed in the International Patent Publications of No. WO 99/34651 or Japanese Patent Publication Hei 6-14556.
An inverter disclosed in WO 99/34651 has a center tap of the primary winding provided with a DC voltage, and semiconductor switches provided between respective ends of the primary winding and the ground so that the semiconductor switches are alternately operated in On-state and Off-state. Further, the inverter is provided with a PWM controller for performing a PWM control so as to supply DC voltage to the inverter. The power supplied from the inverter to the load can be controlled by means of said DC current control performed by the PWM controller.
An inverter of the JP Hei 6-14556 comprises a primary winding having a DC power source connected to a center tap; a secondary winding for outputting an AC voltage; a booster transformer having a tertiary winding for feedback purpose; a resonance capacitor connected between both ends of the primary winding of the booster transformer to constitute an LC resonance circuit between the capacitor and the inductance of the primary winding; a pair of semiconductor switches, one end of each semiconductor switch being connected to the other end of the resonance capacitor, the remaining end of each semiconductor switch being connected to the earth, and the semiconductor switches being alternately operated On-Off states in accordance with a voltage output from the tertiary winding; and a variable inductance connected to the inside of an LC resonance circuit. The voltage output from the inverter is controlled by means of controlling the inductance of the variable inductor.
The inverter using the conventional Royer circuit has difficulty to be down-sized and suffers a problem of low conversion efficiency. The inverter of WO 99/34651 additionally requires a PWM controller for performing the PWM control so as to supply DC voltage to the inverter, of which overall structure of the DC-AC converter ends up being complicated, and difficulty still remains in its down-sizing. As for the inverter of JP Hei 6-14556, since it has a variable inductor connected to the inside of the LC resonance circuit such that an output voltage can be controlled by controlling the inductance of the inductor, the structure of the inverter becomes complicated and difficulty of down-sizing is still unresolved.